It is known that cellular water in growing woody plants in cold districts keep liquid state at low temperature. It is considered that water in xylem parenchyma cell is supercooled down to −40° C. due to the physical property of water, since water in xylem parenchyma cells is isolated from external environment as water droplets (Non-patent reference 1). In other words, cell walls surrounding xylem parenchyma cells act as barriers preventing dehydration from the cells and preventing penetration of extracellular ices into the cells when extracellular ices are generated, and thus intracellular water behaves as water droplets isolated from external environment and is supercooled.
Furthermore, it is suggested that phenol compounds contained in over-wintering plants function as antifreezing substances (Non-patent reference 2).
Still furthermore, it has been disclosed that flavonoids are used for freezing medium for culturing reproductive cells and the like (Patent reference 1) and for cooling liquid of internal combustion engine and the like as ingredients of antifreezing liquid (Patent reference 2).
Additionally, as for flavonoid glycosides as a supercooling promoting agent of the present invention, a various type thereof are present in plants including trees and in substances derived from living bodies as secondary metabolites (Non-patent reference 3).
Patent reference 1: Japanese Patent Application Public Disclosure No. 2000-500327 (WO97/14785)
Patent reference 2: International Publication WO2004/074397
Non-patent reference 1: Kagaku to Seibutsu, vol. 43, No. 5, 280-282 (2005)
Non-patent reference 2: Kagaku to Seibutsu, vol. 37, No. 12, 778-780 (1999)
Non-patent reference 3: “FLAVONOIDS Chemistry, Biochemistry and Applications” published in 2006 by CRC Press Taylor and Francis Group.